1. Field of the Invention
The present invention relates to an exhaust gas recirculation device for removing nitrogen oxide from the exhaust gas of an internal combustion engine and control method thereof.
2. Description of the Related Art
Conventionally, exhaust gas recirculation (EGR) for returning a part of exhaust gas to intake side is used for reducing nitrogen oxide, so-called NOx, contained in the exhaust gas of an internal combustion engine, especially a reciprocating engine.
According to the exhaust gas recirculation, a gas having a large thermal capacity, such as carbon dioxide and water vapor, is mixed into combustion air supplied to the internal combustion engine to increase thermal capacity of the air, so that maximum combustion temperature is lowered, thus decreasing the level of nitrogen oxide.
An example of an internal combustion engine using the exhaust gas recirculation is shown in Japanese Utility Model Laid-Open Publication No. Hei 5-30454. According to the exhaust gas recirculation system in the publication, a part of the exhaust gas of the engine is returned to the intake side of a supercharger.
Accordingly, even when exhaust gas pressure is insufficient for returning the part of the exhaust gas, the part of the exhaust gas is securely fed to intake side of the engine by suction force of the supercharger, so that the recirculation rate can be improved.
Another internal combustion engine having high-load EGR duct and low-load EGR duct is shown in Japanese Patent Laid-Open Publication No. Hei 6-229326.
The high-load EGR duct introduces a part of the exhaust gas from downstream of catalyst to return to intake side of the supercharger. On the other hand, the low-load EGR duct introduces a part of the exhaust gas from an exhaust manifold and returns the exhaust gas to around the intake port of respective cylinders.
Accordingly, the high-load EGR duct and the low-load EGR duct can be switched in accordance with the load applied to the engine, so that a more appropriate amount of the exhaust gas can be recirculated as compared to one EGR duct.
In the internal combustion engine employing exhaust gas recirculation, when differential pressure on the exhaust side and the intake side fluctuates, because flow rate of the exhaust gas flowing in the exhaust gas recirculation duct also fluctuates, the recirculation rate cannot be controlled at a constant level.
Accordingly, a predetermined amount of the exhaust gas does not recirculate to the intake side even when the recirculation amount is set in accordance with operation of the internal combustion engine, so that it is difficult to set an optimum amount of exhaust gas in accordance with the internal combustion engine.
A flow control valve maybe provided to the exhaust gas recirculation duct to control the flow rate of the exhaust gas flowing through the exhaust gas recirculation duct. However, the flow control valve increases the flow restriction of the exhaust gas recirculation duct, so that necessary recirculation cannot be obtained when differential pressure between the exhaust side and the intake side is low.
Especially, in a diesel engine having a supercharger, the pressure on the exhaust gas side can be higher than the intake side according to an operation condition, thereby making it impossible to recirculate the exhaust gas to the intake side.
An object of the present invention is to provide an exhaust gas recirculation device for recirculating an appropriate amount of exhaust gas in accordance with an operation condition of an internal combustion engine and a control method thereof.
A device according to the present invention is an exhaust gas recirculation device for extracting a part of an exhaust gas of an internal combustion engine and recirculating the exhaust gas into an intake side, the exhaust gas recirculation device having a gas feeder installed on a recirculation duct connecting an exhaust duct and an intake duct of the internal combustion engine.
As the gas feeder, an airblower capable of pumping gas through narrow space such as a duct and a compressor for compressing fluid inhaled through an air intake to blow out from an air outlet can be used.
For instance, a Roots blower, an axial-flow air blower and a centrifugal air blower can be used for the air blower as the gas feeder.
A reciprocating compressor, a screw compressor, a vane compressor, a scroll compressor, an axial-flow compressor and a centrifugal compressor can be used for the compressor as the gas feeder.
A drive source for driving the gas feeder may be a drive source independent from the internal combustion engine, such as a direct-current motor, an alternating-current motor, a hydraulic motor and an air motor, or may be the internal combustion engine itself.
Accordingly, because the part of the exhaust gas is forcibly pumped to the intake side by the gas feeder, even when differential pressure between the exhaust side and the intake side fluctuates, the flow rate of the exhaust gas flowing through the exhaust gas recirculation duct can be adjusted to a constant level by appropriately controlling the drive source for driving the gas feeder, so that the recirculation amount can be controlled to make the recirculation rate constant.
Therefore, even when the pressure on the intake side is than the exhaust side, a predetermined amount of exhaust gas can be recirculated to the intake side by setting an optimum amount of recirculation in accordance with an operation condition of the internal combustion engine and controlling the drive source to recirculate the preset recirculation amount.
In the device of the present invention, at least one of an air mixer for mixing outside fresh air with the exhaust gas recirculating to the intake side, a water mixer for mixing water with the exhaust gas recirculating to the intake side, and a cooler for cooling the exhaust gas recirculating to the intake side may preferably be provided between the gas feeder and an exhaust side of the internal combustion engine.
By providing the air mixer, the high-temperature exhaust gas is mixed with low-temperature air, thus lowering the temperature of recirculating exhaust gas. The outside fresh air may be the air directly inhaled from the outside of the internal combustion engine, or alternatively, may be a part of intake air introduced from air intake for combustion.
Using the water mixer, the temperature of the high-temperature exhaust gas can be lowered by mixing low-temperature water and the temperature of the recirculated exhaust gas can be further lowered by heat of evaporation when the water is evaporated.
The cooler may be a liquid-cooling type for cooling the exhaust gas by a cooling medium such as water, an evaporative cooling type for cooling the exhaust gas by the heat of evaporation of liquid, or an air-cooling type for cooling the exhaust gas by ambient air, which cools the high-temperature exhaust gas to lower the temperature of the recirculated exhaust gas.
According to the above, the high-temperature exhaust gas does not flow into the gas feeder, so that damage to the gas feeder by heat can be prevented and, furthermore, because the temperature of the intake air inhaled by the internal combustion engine does not get too high, so that the combustion temperature can be securely curbed by the exhaust gas recirculation.
In the device of the present invention, the gas feeder, a drive source for driving the gas feeder and the cooler may preferably be integrated as a unit.
Accordingly, the device can be easily attached and detached to the internal combustion engine. Therefore, the exhaust gas recirculation device can be easily installed to a conventional internal combustion engine having no exhaust gas recirculation device.
When the internal combustion engine is run at a place with thin air such as a place at a high altitude where the internal combustion engine does not work stably at a high load with exhaust gas recirculation, the exhaust gas recirculation device unit can be detached from the internal combustion engine, so that the internal combustion engine can be operated stably at high load. Incidentally, the nitrogen oxide in the exhaust gas can be reduced at the place with thin air even without the exhaust gas recirculation.
On the other hand, the nitrogen oxide contained in the exhaust gas of the internal combustion engine can be reduced in a place of ordinary air pressure by attaching the exhaust gas recirculation device unit to the internal combustion engine.
In the device of the present invention, the gas feeder may preferably be capable of rotating in both normal and reverse directions, a rotation in the normal direction circulating fluid in reverse to a rotation of the reverse direction, and a controller for controlling the gas feeder may preferably be provided, the controller controlling rotary direction of the gas feeder in accordance with an operation condition of the internal combustion engine and reversely rotating the gas feeder when the exhaust gas is not recirculated to the internal combustion engine.
The gas feeder may be a Roots blower, an axial-flow air blower, a screw compressor or a vane compressor.
Accordingly, when the exhaust gas recirculation is not conducted, even if the pressure at the exhaust side is greater than that of the intake side, the flow from the exhaust side to the intake side can be prevented by the reverse rotation of the gas feeder.
Accordingly, the control valve for preventing the flow from the exhaust side to the intake side is not necessary to be provided to the recirculation duct, so that an increase in the flow restriction by the provision of the control valve and decrease in exhaust gas recirculation amount in accordance with an increase in the flow restriction can be prevented in advance.
Further, by the reverse rotation of the gas feeder, when the recirculating exhaust gas is cooled by the cooler, soot and contaminant stuffed in the cooler are discharged to the exhaust side, thus cleaning the cooler.
In the device of the present invention, the gas feeder may preferably a Roots blower.
Because the Roots blower can be operated at a relatively low speed, the size of the drive source can be made small. Further, because the size of the Roots blower itself is relatively small, an increase in the size of the exhaust gas recirculation device can be prevented.
Because the flow characteristics of the Roots blower are similar in both the normal rotation and the reverse rotation, the relationship between the rotation speed and the air feed amount is similar in both the normal rotation and the reverse rotation except for the rotary direction, thus easily preventing backflow and conducting control of the rotation speed during cleaning the cooler.
In the device of the present invention, a catalyst for cleaning the exhaust gas may preferably be provided to the exhaust duct of the internal combustion engine, and a part of the exhaust gas may preferably be extracted from a downstream of the catalyst to be recirculated.
Accordingly, because the exhaust gas is extracted from a downstream side of the catalyst, the exhaust gas returning to the intake side is cleaned and the temperature of the exhaust gas is reduced, thus decreasing damage and abrasion of the equipment installed on the recirculation duct such as the gas feeder and, when the cooler is installed in the recirculation duct, the volume of the cooler can be reduced.
In the device of the present invention, the gas feeder may preferably be capable of adjusting air feed flow rate, and the exhaust gas recirculation device may preferably have an engine speed detector for detecting an engine speed of the internal combustion engine, a fuel supply detector for detecting fuel supply to the internal combustion engine, a memory for storing a map indicating a rotation speed of the gas feeder capable of obtaining an optimum exhaust gas recirculation amount in accordance with the engine speed of the internal combustion engine and the fuel supply to the internal combustion engine, and a controller for adjusting the air feed flow rate of the gas feeder based on the map stored in the memory in accordance with an operation condition of the internal combustion engine.
Accordingly, the engine speed and the fuel supply amount can be detected by the engine speed detector and the fuel supply detector. By acquiring the rotation speed of the gas feeder capable of obtaining the exhaust gas recirculation amount in accordance with operation condition of the internal combustion engine relative to the engine speed and the fuel supply through experiment and calculation, a map indicating an optimum rotation speed relative to the engine speed and fuel supply can be obtained.
By providing a memory for storing the aforesaid map, the rotation speed of the gas feeder can be rapidly adjusted so that the optimum exhaust gas recirculation amount can be obtained, thus securely recirculating the optimum amount of exhaust gas in accordance with an operation condition of the internal combustion engine.
A method of the present invention is a control method of an exhaust gas recirculation device for extracting a part of exhaust gas of an internal combustion engine to recirculate to an intake side. The method includes the steps of: providing to the exhaust gas recirculation device a gas feeder installed to a recirculation duct for connecting an exhaust duct and an intake duct of the internal combustion engine and a drive source for driving the gas feeder, the gas feeder controlling a rotation speed of the drive source to control air feed flow rate; and controlling a rotation speed of the drive source to obtain an optimum exhaust gas recirculation amount in accordance with operation condition of the internal combustion engine.
According to the above, by employing the gas feeder, such as a Roots blower, capable of adjusting flow rate by adjusting rotation speed thereof, the exhaust gas recirculation amount sent to the intake side of the internal combustion engine can be easily adjusted by adjusting the rotation speed of the drive source for driving the gas feeder. Further, by employing an electric motor such as direct current motor, the rotation speed can be easily and rapidly changed, thus ensuring appropriate exhaust gas recirculation amount in accordance with operation condition of the internal combustion engine.
Another method of the present invention is a control method of an exhaust gas recirculation device for extracting apart of exhaust gas of an internal combustion engine to recirculate to an intake side, the method having the steps of: providing to the exhaust gas recirculation device a gas feeder installed to a recirculation duct for connecting an exhaust duct and an intake duct of the internal combustion engine and a drive source for driving the gas feeder, the gas feeder being capable of rotating in both normal and reverse directions, rotation in the reverse direction circulating fluid in a direction reverse to rotation in the normal direction; and reversely rotating the gas feeder to prevent backflow from the intake side to the exhaust side when the exhaust gas is not recirculated to the internal combustion engine.
Accordingly, as described in the aforesaid device of the present invention, the backflow from the intake side to the exhaust side can be prevented by the reverse rotation of the gas feeder when the exhaust gas is not recirculated to the internal combustion engine and the recirculation duct including the cooler can be cleaned.